In the definition of RACF (Resource Admission Control Function) in Y.2111 of the NGN standard organization of ITU-T (International Telecommunications Union-Telecommunication Standardization Sector), a RACF system is described as including two control function entities: a PD-FE (Policy Decision Function Entity) and a TRC-FE (Transport Resource Control Function Entity). Meanwhile, this standard also defines two transport function entities: a PE-FE (Policy Enforcement Function Entity) and a TRE-FE (Transport Resource Enforcement Function Entity), both of which belong to the transport function layer. Y.2111 describes that there are requirement for one-to-multiple and multiple-to-one correspondence between PD-FE and PE-FE, PD-FE and TRC-FE and also requirement for interconnection between multiple examples of each function entity.
As for how to discover the presence of the other side for these function entities, Y.2111 does not provide any corresponding description, nor describe the method that should support dynamic discovering. In a common situation, we can adopt means of static configuration such as SNMP (Simple Network Management Protocol) to enable a function entity to discover the presence of a neighbor, who may be the same function entity as itself or another function entity. The method of static configuration has defects including requiring corresponding work of network management, unable to know a neighbor's change quickly and in time, and unable to know the relationship of interconnection of automatic update quickly and in time, which will cause a wrong decision on transport function control.